Curious about the lowest choice of screen brightness that might ever be chosen by someone working with photos. 80 cd/m^2 is a common recommendation for editing in a dark room, but is there ever a reason to use a darker screen, say 40 or 20 cd/m^2?
No. The purpose of a viewing screen is to emulate the human vision system and make it easier for you to discern details. If you're going to go to print then you need a screen that can show you or emulate the approximate details of that print.
I used to work with screens that were calibrated for radiological purposes. Think medical. Their calibrations were around 0.5 cd/m2 on the low end up to 100 cd/m2 on the high end. However they really needed to be calibrated up to about 300cd/m2 to take full advantage of how the eye perceives both color and luminance.
Going outside of the range simply blocks up the highlights and shadows, and puts additional strain on your eye as it adjusts to the large differences.
The rest of the room also has an impact on your ability to adjust and perceive color, so keeping that 'in spec' as well is important. It'll make your editing day easier.